exo·​sphere ˈek-sō-ˌsfir How to pronounce exosphere (audio)
: the outer fringe region of the atmosphere of the earth or a celestial body (such as a planet)
exospheric adjective

Example Sentences

Recent Examples on the Web Meanwhile, in the low exosphere, the environment continues to degrade. Raffi Khatchadourian, The New Yorker, 21 Sep. 2020 About four hundred miles above the Earth, the exosphere begins. Raffi Khatchadourian, The New Yorker, 21 Sep. 2020 The mysterious moon has a very, very thin atmosphere, called an exosphere, that’s rich in carbon dioxide, and trace amounts of oxygen and nitrogen. Jennifer Leman, Popular Mechanics, 7 Apr. 2020 Mercury has what’s called an exosphere made from solar-wind propelled oxygen, sodium, hydrogen, helium, and potassium atoms that have bounced off the planet's surface. Jennifer Leman, Popular Mechanics, 9 Nov. 2019 The problem is that if there really are 1,000 ions per cubic centimeter, the ionization of gas in the lunar exosphere can’t account for such a high concentration—there just isn’t enough gas. Daniel Oberhaus, WIRED, 16 July 2019 In the 1970s two Soviet lunar orbiters discovered that ions also existed in the moon’s ultra-thin exosphere, and scientists have been trying to explain this observation ever since. Daniel Oberhaus, WIRED, 16 July 2019 Instead scientists refer to its extremely thin covering of oxygen, hydrogen, sodium, helium, and potassium as an exosphere. National Geographic, 27 Mar. 2019 Because nitrogen is a volatile chemical, some of it sublimates and gives Pluto a tenuous atmosphere still thick enough to be more than an exosphere. John Wenz, Popular Mechanics, 17 July 2015 See More

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'exosphere.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

Word History


exo- + -sphere

First Known Use

1949, in the meaning defined above

Time Traveler
The first known use of exosphere was in 1949

Dictionary Entries Near exosphere

Cite this Entry

“Exosphere.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/exosphere. Accessed 8 Dec. 2022.

Kids Definition


exo·​sphere ˈek-sō-ˌsfi(ə)r How to pronounce exosphere (audio)
: the outermost region of the atmosphere of the earth or a planet

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